Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charge-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Many electronic devices are powered by voltage supplies that are not adequate for the semiconductor devices or other components within the electronic device. For instance, some electronic parts require 9 volts or 12 volts to operate, but are in an electronic device powered by two 1.5-volt batteries in series or 5 volts from a universal serial bus (USB) port. In other cases, a device is powered by a 5 or 12 volt source, but needs between 20 and 90 volts for a certain circuit element, e.g., an avalanche photodiode (APD).
In cases where input voltages need to be converted for powering of components, switch-mode power supplies (SMPS) are commonly used. Boost regulator 10 in FIG. 1 is one example of an SMPS topology. Boost regulator 10 receives an input voltage at VIN node 12, and converts the voltage to an output voltage at VOUT node 14. Electrical current flows through boost regulator 10 in two different paths. The first path is through inductor 18 and MOSFET 20 to ground node 24, and the second path is through inductor 18 and diode 26 to VOUT node 14. Electrical current flows primarily through the first path when MOSFET 20 is turned on. The electrical current through inductor 18 stores energy in the inductor through magnetization of a ferric core. The electrical current through inductor 18 reaches a fairly high magnitude because MOSFET 20 creates a substantially short circuit between VIN node 12 and ground node 24 through inductor 18.
When a sufficient current through inductor 18 is reached, MOSFET 20 is shut off, and current flows instead through inductor 18 and diode 26 to VOUT node 14. The energy stored magnetically in inductor 18 during the first phase results in an electrical current through diode 26 larger than what load 16 would normally draw. The excess current through diode 26 raises the voltage potential at VOUT node 14 above the voltage potential of VIN node 12. Capacitor 28 stores electrical charge not immediately usable by load 16. As load 16 uses up the electrical energy stored in capacitor 28, the voltage potential of VOUT node 14 drops. When the voltage of VOUT node 14 drops below a desired threshold, MOSFET 20 turns on and off again to inject more energy through diode 26 to VOUT node 14.
To determine when MOSFET 20 should turn on and off, a resistor voltage divider is formed from resistors 30 and 32 to create a feedback voltage at VFB node 34. The voltage potential at VFB node 34 is given by equation 1.
                              V          FB                =                                            V                              OUT                ⁢                                                                                        *                          R              32                                                          R              30                        +                          R              32                                                          Equation        ⁢                                  ⁢        1            
The voltage potential at VFB node 34 is proportional to the voltage potential at VOUT node 14, but reduced by the ratio of resistors 30 and 32. Comparator 40 includes a first input coupled to VFB node 34, and a second input coupled to a reference voltage at VREF node 42. Comparator 40 has an output 44 that indicates whether the voltage potential of VFB node 34 is above or below the voltage potential of VREF node 42.
Control logic 50 receives the output signal 44 from comparator 40, and turns MOSFET 20 on and off using control signal 52 based on comparator output signal 44. Several methods exist for controlling MOSFET 20 based on output signal 44. In some embodiments, control logic 50 will increase a switching frequency or duty cycle of control signal 52 when VFB falls below VREF. In other embodiments, MOSFET 20 is switched at a predetermined frequency when VFB node 34 is below VREF node 42, and remains off when VFB node 34 is above VREF node 42.
An engineer designing a power supply based on voltage divider feedback generally buys a controller IC with comparator 40, VREF 42, and control logic 50. Given VREF 42 set by the controller IC manufacturer, the engineer selects resistance values of resistor 30 and resistor 32 to set the voltage potential that VOUT node 14 will be regulated to. The voltage potential at VOUT node 14 will settle at a voltage indicated by equation 2.
                              V          OUT                =                                            V              REF                        *                          (                                                R                  30                                +                                  R                  32                                            )                                            R            32                                              Equation        ⁢                                  ⁢        2            
Using a resistive voltage divider for feedback creates a voltage at VFB node 34 that is acceptable for input to comparator 40. However, the resistive voltage divider is not a flexible approach. The voltage divider is not easily calibrated to provide high-resolution voltage control in a limited voltage range of VOUT node 14. Moreover, the voltage divider not only reduces voltage from VOUT node 14 to VFB node 34, but also reduces the magnitude of changes in voltage. The reduction in magnitude of voltage changes from VOUT node 14 to VFB node 34 makes the control circuitry less robust against noise.
Therefore, a need exists for a more flexible feedback mechanism for voltage regulators.